Sturdy shoes first came into widespread use between 40,000 and 26,000 years ago, according to a US scientist.

Humans' small toes became weaker during this time, says physical anthropologist Erik Trinkaus, who has studied scores of early human foot bones.

He attributes this anatomical change to the invention of rugged shoes, that reduced our need for strong, flexible toes to grip and balance.

The research is presented in the Journal of Archaeological Science.

The development of footwear appears to have affected the four so-called "lesser" toes - excepting the big toe.

Ancient footwear

While early humans living in cold northern climates may have begun covering up their feet to insulate them as early as 500,000 years ago, protective footwear comparable to modern-day shoes is thought to be a much later innovation.

It has been difficult for archaeologists to determine exactly when humans stopped going barefoot, however, because the plant and animal materials used to make prehistoric shoes is highly perishable.

"The oldest shoes in the world are about 9,000 years old, and they're from California," said Professor Trinkaus, of Washington University in St Louis, US.

But by examining the foot bones of early modern humans (Homo sapiens) and Neanderthals (Homo neanderthalensis) dating from 10,000 to 100,000 years ago, Professor Trinkaus says he has determined the period in which footwear became the norm.

Anatomical evidence

He found Neanderthals and early moderns living in Middle Palaeolithic times (100,000 to 40,000 years ago) had thicker, and therefore stronger, lesser toes than those of Upper Palaeolithic people living 26,000 years ago.

To test the theory that the more delicate toes resulted from shoe use, the Washington University researcher compared the foot bones of early Native Americans, who regularly went barefoot, and contemporary Alaskan Inuits, who sported heavy sealskin boots.

Again, he identified chunkier toes in the population that routinely went without shoes. The research suggests shoe-wearers developed weaker toes simply because of the reduced stresses on them during their lifetime; it was not an evolutionary change.

The comparison proves his hypothesis, he says: "It has been suggested in the past that thicker toes and fingers are related to greater blood supply in colder climates, but it just doesn't hold up."

Cultural "explosion"

The advent of footwear occurred during a period Professor Trinkaus describes as "a well-documented archaeological explosion" which also produced a number of other notable human advances.

Paul Mellars, professor of prehistory and human evolution at the University of Cambridge, UK, agrees there were "dramatic changes" in human behaviour at this time. "From 35,000 years ago onward, you see the first art, the first stone tools, and the first personal decorations and jewellery."

More advanced shoe-making skills could have been a product of this overall increase in technological ingenuity.

"There is a strong hint that people were doing more complicated things with ...skins, with special stone tools for cleaning and awls for piercing.

"In view of all these changes, it wouldn't be at all surprising if we saw better shoes," Professor Mellars explained.